This invention pertains to semiconductor devices with integrated sensors and more particularly to monolithic semiconductor devices with integrated surface micromachined structures.
Semiconductor devices with integrated sensors are of great interest because of their potential of reduced cost and size in applications requiring the sensing of, for example, acceleration, flow or pressure. For many years, discrete micromachined sensors have been commercially available. These discrete sensors have almost exclusively been fabricated by bulk micromachining techniques, that is the silicon substrate is machined to form the sensing element. While the techniques that have been developed have produced reliable devices, such devices are typically large and not easily integrated with modern semiconductor integrated circuit (IC) device processing. As a result, surface micromachining, that is micromachining of layers deposited on the surface of a substrate, has been the subject of much recent investigation.
Surface micromachining offers the potential of manufacturing much smaller sensors than are possible with bulk micromachining sensor technology. This potential could provide for integrating such sensors with IC devices on a single chip, thus producing a monolithic semiconductor device. Inherent advantages of a monolithic device include improved reliability and lower cost. These integrated devices additionally provide higher sensitivity and accuracy, as well as better compensation of side effects and data acquisition than discrete devices.
A particular problem is that surface micromachined devices have proven to be extremely dependent on the particular processing scheme employed. Often the best process for fabricating the sensor is in direct conflict with that for fabricating the IC device. For example, the high temperature (900 degrees centigrade or higher) IC processes, such as field oxidation or source/drain drive, can create stresses in the relatively thick (1 to 2 micron) polysilicon layers required for surface micromachined structures (SMS's). Such stress accumulation due to multiple high temperature processes can result in deformation of the sensor structure.
Recently, Steven J. Sherman, et al. in U.S. Pat. No. 5,417,111 "MONOLITHIC CHIP CONTAINING INTEGRATED CIRCUITRY AND SUSPENDED MICROSTRUCTURE", issued May 23, 1995, has suggested a solution to this integration problem that include either bipolar circuit elements or metal on silicon (MOS) circuit elements, or circuits that combine both types of elements, BiMOS. Specifically, Sherman et al. propose a process in which the bipolar and/or MOS circuit elements are created before any SMS's thus avoiding the previously mentioned temperature problem. However, this processing sequence imposes limitations to process optimization of the SMS's, such as limiting the ability various layer thickness' or to use thermal anneal and doping cycles to create optimized SMS's, due to the presence of preformed IC circuit elements. The heat of such thermal anneal and doping cycles can degrade the performance of such circuit elements.
Thus it would be beneficial to devise a new process for fabricating monolithic semiconductor devices with integrated surface micromachined structures having both optimized IC and SMS elements.
It is therefore a purpose of the present invention to provide a new process for fabricating monolithic semiconductor devices having both optimized IC and SMS elements.
It is another purpose of the present invention to provide a new process for fabricating monolithic semiconductor devices with integrated surface micromachined structures that provides devices with improved reliability and lower cost.
It is still another purpose of the present invention to provide a new and improved method of fabricating monolithic semiconductor devices with integrated surface micromachined structures that provides devices with higher sensitivity and accuracy as well as better compensation of side effects and data acquisition.